The molecules involved in cell movement and changes of cell shape assume a variety of supramolecular forms during the cell cycle. For example, actin exists as monomers, filaments, and filament-bundles. These assemblies are transient and their formation must be under strict cellular controls. The filament-bundles appear to be part of a membrane-associated cytoskeleton which may be related not only to cell movements and cell shape, but also to cell adhesion and contact inhibition. Our approach is to purify the components involved in these cellular events and to study them biochemically and structurally, with emphasis on factors controlling their interaction and their assembly into higher order supramolecular forms. We have extensively purified and characterized actin and myosin from amoeba of Dictyostelium discoideum. Purification and characterization of various factors that affect actin assembly is underway. Furthermore, we have recently discovered that phosphorylation of Dictyostelium myosin heavy chain inhibits the assembly of myosin into bipolar thick filaments. The cellular controls acting on this phosphorylation of myosin are currently being examined.